1. Field of the Invention
The present invention relates to a cathode-ray tube having a face panel.
2. Description of the Related Art
The envelope of a cathode-ray tube generally comprises a substantially rectangular face panel having an inner surface on which a phosphor screen is formed, and a funnel coupled with the face panel by frit glass and the like. The funnel has a neck portion extending to a side opposite to the face panel, and an electron gun assembly is incorporated in the neck portion.
The outer and inner surfaces of the face panel are formed in a curved shape whose central portion projects outside such that distances between the deflection center of electron beams emitted from the electron gun assembly and a large number of scanned positions on the phosphor screen are set to be equal to each other as possible. The shape of the outer surface of the face panel is a very important factor which influences the performance of the cathode-ray tube itself and the visual impression of the cathode-ray tube.
In recent years, as a method of expressing the shape of the outer surface of a face panel, the following method is often used. That is, by using orthogonal coordinates which uses as the origin O, the center of the outer surface of the face panel, as an X-axis, a horizontal axis passing through the origin O and perpendicular to a central axis (Z-axis) of the envelope, and as a Y-axis, a vertical axis passing through the origin O and perpendicular to the Z-axis are used, and a decent amount z of an arbitrary point (x,y,z) on the outer surface is given by the following polynomial: ##EQU2##
where i, j and n are integers of zero or more, and a.sub.ij are predetermined coefficients.
When the shape of the outer surface of the face panel is to be defined by the above polynomial, setting of four coefficients a.sub.10, a.sub.20, a.sub.01, and a.sub.02 of the coefficients a.sub.ij is most important. The coefficients a.sub.10 and a.sub.20 are coefficients for determining a curved shape along an x-axis (horizontal direction) of the outer surface of the panel, and the coefficients a.sub.01 and a.sub.02 are coefficients for determining a curved shape along a y-axis (vertical direction) of the outer surface of the panel. The substantially entire curved surface of the outer surface of the face panel is determined by the above four coefficients.
When the above four coefficients are improperly set, the following problem is posed. That is, when external light, e.g., light from fluorescent lamps in a room, is reflected on the outer surface of the face panel, the shape of the reflected external light image is unnaturally distorted, and the distorted image makes a user feel visually uncomfortable. In a conventional technique, in order to prevent external light from being reflected on the face panel, a special chemical treatment is performed to the outer surface of the face panel to cause the outer surface to be rough. The above surface treatment degrades the basic performance of a cathode-ray tube, e.g., resolution, and the manufacturing cost of the cathode-ray tube is increased by increasing the number of the steps in manufacturing the cathode-ray tube.